Why cross your finger?

Fantasy of Crossed Fingers
(Psychology)

To cross one's fingers is a hand gesture used to superstitiously wish for good luck or to nullify a promise. The supposed loophole of crossing one's fingers whilst making a promise is usually exploited by school-aged children.
Crossing one's fingers, by curling the middle finger over the index, is thought to bring good luck. This dates back to when crossed fingers were used as a gesture to ward off witches and others considered to be or possessed by evil spirits. It is also seen as bad luck to cross your fingers on both hands.

Some believe that the gesture originates from pre-Christian times and, in many early European cultures, two people were required to use their index fingers to form the sign, one to make a wish and the other to support it. It was believed that the cross was a symbol of unity and that benign spirits dwelt at its intersecting point—to wish on a cross was a figurative way of securing the wish at the intersection until it came true. Over the years, the custom was modified so one person could make a wish on his/her own.The 'crossed fingers' gesture is used for the logo of the United Kingdom's National Lottery, as well as the Oregon Lottery and Virginia Lottery in the United States.Crossed fingers behind the back can also be used when making a promise or a claim to indicate that you are lying. This has been used in television and movies to clue in the viewers while keeping other characters in the dark.

In some countries, other gestures symbolize the wish for luck, e.g. in Germany and Poland "pressing thumbs" means to wish somebody luck.

Courtesy:Wikipedia